Bad advice frustrates!

[g__day (Matthew)]

After a raging battle trying to successfully drift align my permanently mounted mount, I sat down with a pencil and paper and thought through the advice I was getting to reach a stunning conclusion - it was wrong!

A quick google on "drift alignment southern hemisphere" brought up alot of links, and it was interesting to see how they described correcting for lattitude - about 1/3 inverted the direction you should take to correct DEC drift, and about 1/3 where poor in describing direction of drift.

I am pleased to say the article on IIS got it right, but talk about tear your hair out following incorrect advice and thinking you are going batty!

As the drift kept getting bigger I simply could not deny what I was seeing. Too a video on drift aligning in the Northern hemisphere had mentioned the direction of lattitude corrections should always take the star back towards its initial position - something that was not occuring with the advice I had printed in black and white to hang on my Astronomy lab wall.

I think from now I will list all the sites that get it wrong and tell them!

The other annoying thing is when a site says if the drift is down or up - instead of North or South. You can ignore movement in RA - so judging from the Central dot of a camera's viewing finder if I see drift North and up - do I compensate the same way if I see drift that is North and down? To my simple mind up / down is West / East - which is to be ignored. North versus South drift is hard to misunderstand!

Firm in my mind now is point East and low to correct elevation mis alignment. Any drift North = raise mount, whilst drift South = lower mount. That plain English would have saved me alot of stress!

Simply maths - what would I see if the mount was at say 10 degrees facing due East at low elevation (instead of elevated to 33 degrees) lead me to understand the advice I had in black and white was wrong.

Isn't it funny how something authorative sound in print is can be treated like holy writ? I remember an Australian Astronomy website that had rough polar alignment advice saying true South was abut 11 degrees West of magnetic South for Sydney. I asked my dad - a retired surveyor and he said no its the opposite - due East. A google again identified two points of view - which I alerted the website owner too - he checked and corrected his advice!

How many other folks have come across inverted advice - should we make a list of mistakes or incorrect thinking to help beginners? I didn't realise quality control was missing on such a fundamental peice of getting started!

[netwolf]

I know exactly what you mean, but fortunately the first time i tried on my own i found the IIS guide. The other good things with the IIS guide is that it tells you how to work out your N-S E-W lines first, most others do not.

I think though for such fundamental information guides should be in a Wiki format. So others are able to correct the information. This method provides better quality control. Sure you will get the random incorrect influence but the experience of the majority will fix it. A Wiki dedicated to Amateur Astronomy techniques and procedures would be awesome. There is a lot of good and bad information out there that i have come across.

Regards
Fahim

[jase (Jason)]

I hear you Matt, its disappointing when the advice provided is not correct. However I feel in this case it comes down to experience. Obviously if you're make Az adjustment and the star drifts faster, then you've got to adjust the other way. The best thing you can do is make large adjustments, then slowly progress to smaller, precise movements. This allows you to get a feel for how far you need to make an adjustment to counter act the drift. Its simply trial and error. After several nights under your belt, drift align becomes "childs play"/thoughtless process.

Of course, we've discuss this before but you can also use software to assist in polar alignment. It typically improves the accuracy by a considerable amount when drift aligning with a camera.

[netwolf]

Jase,

Any particular software you recommend?

Regards
Fahim

[g__day (Matthew)]

Jase -

I did it a crude way - both axes at once, pointing South East at 45 degrees - and got 900 second unguided images as an end result and spot on pointing. But I felt I could do better with doing it the right way - and went backwards and side ways. Mainly because I expected problems and strangeness and was willing to follow the book - until it started getting farical.

Too I tried PEMPro V2 and found its drift align gave me wierd behaviour and instructions - most likely because 1) it can't tell ASCOM to halt tracking for 15 seconds to get scale right on my mount (why it doesn't just pulse guide to slow RA to 0 is beyond me) and 2) I didn't compensate for this limitation correctly!

MaximDL I think can drift align to - I'm still investigating!

[jase (Jason)]

Fahim,
Before I digress, such software requires a camera. The software will not help for visual aligning.

You can actually use any auto-guiding software that logs corrections in X (RA) and Y (DEC) axis. Simply log the output and pipe it into excel to graph the result. There are many instructions to do this, but I've found Brad Moore's to be the clearest. His instructions and excel template can be found here: http://www.southern-astro.com.au/php...preciseCCD.php

If you're not into doing the ground work, then I'd recommend two products, both of which I've used: PolarAlignMax and PEMPro.v2

PolarAlignMax (from the producers of the legendary FocusMax) requires a full version of the Astrometric plate solving engine known as PinPoint. PinPoint LE (light edition or something) comes with MaximDL, but does not support the full plate solving features required. PolarAlignMax works by taking a series of images and plate solves them down to sub arcsecond accuracy. Based on the offset of these images, it will determine how far from the true pole you are and advise what needs to changed to correct the alignment error. It continues to do this iteratively until you reach your desired accuracy. Reasonably simple to use, though I've had a few minor issues with plate solving due to the FSQ's FOV. I used the USNO-2.0 (6GB) catalogue that will solve stars down to 18th mag. There were times where the platesolve would catalogue ~34,000 stars in a single frame and fail. This is way to many. Simply 100-200 stars is more than sufficient. I have since migrated to the GSC (Guide Star Catalogue 1.1 ACT (400Mb)) which does not go as deep (and altered the catalogue search criteria). Note, you'll need to output in FITS format for plate solves to work as the FITS header contains the RA/DEC of the image center. This give the plate solving engine a starting point to solve from.

PEMPro... This would have to be the Swiss army knife for fine tuning your telescope mount. An incredible piece of software. Now, version two, has a cool drift alignment tool which takes the guess work out of drifting. Dare I say it, but the drift alignment feature is actually "fun" to use. Almost too easy to use. The added benefit, is once you've finished drifted aligning, why not also go tune your mounts PE and Backlash. Three tools for the price of one. Just make sure you read the software requirements to ensure you've got compatible hardware/software. I use this exclusively now for drifting and PEC training.

Of course, you can also polar align your mount by building a pointing model (say 100 or so points). This can be done using TPoint or MaxPoint, but your costs will begin to increase significantly with these solutions. They are perhaps the most accurate as they also take into consideration other anomalies such as mirror flop, flexure etc.

Presented are a few options, I feel sure others may have found other software that works well.

[jase (Jason)]

Originally Posted by g__day

Jase -

I did it a crude way - both axes at once, pointing South East at 45 degrees - and got 900 second unguided images as an end result and spot on pointing.

But what more do you want Matt? 15mins unguided with pin point stars is nothing to be laughed at. You'd probably get 20 or 30 mins from this level of alignment when guiding. May show slight field rotation for the latter duration, but how many times are you going to go over 20 min subs. For a uncooled DSLR, I'd say never. The noise would be too great. For a cooled CCD camera with a 3nm or 6nm Ha filter, I'd say its a likely possibility.
Seriously, there comes a time when close enough is near enough for the type of work you are performing. It great to strive for perfection, but when you start tearing your hair out, you know you've taken it too far.

Originally Posted by g__day

Too I tried PEMPro V2 and found its drift align gave me wierd behaviour and instructions - most likely because 1) it can't tell ASCOM to halt tracking for 15 seconds to get scale right on my mount (why it doesn't just pulse guide to slow RA to 0 is beyond me) and 2) I didn't compensate for this limitation correctly!
MaximDL I think can drift align to - I'm still investigating!

Thats not a problem with PEMPro. If ASCOM can't stop the mount tracking automatically, you can simply stop it manually via the hand controller/paddle. In fact, the PEMPro instructions tell you to do this. Place the mount in terrestrial mode or similar will do the trick. If calculating the scale is all you need you can do this with some simple math, you don't need to let PEMPro do it. Follow the wizard for instructions - http://www.siriusimaging.com/Help/PE...lignwizard.htm

Yes, you can drift align with MaximDL, CCDSoft etc etc, any program that will log guiding corrections will work. See my previous post to Fahim.

[ballaratdragons (Ken)]

There are thousands and thousands of sites with a mistake. They all still call Pluto a 'Planet'

I agree with them, but it isn't correct any more!

Oops, that reminds me, I must correct it on my own site

[g__day (Matthew)]

Jase,

Tracking wasn't consistently that good when the scope was close to vertical - and without better knowledge I tried to do better!

As a permanent pier I want to get this as close to the SCP as I humanly can - then forget all about it and move onto the next variable and then the next etc!

Yes I know it was my crude usage of PEMPro V2.0.37 that stuffed things - it and I aren't an idiot proof combination yet!

Possibly I need to adjust my mount to the SCP then lock it tighter to the pier - but mt pier design didn't allow for this - some surgery (angle grinder) is needed to get a spanner inside a 140mm diameter tube! When I push on the head of the pier stars move 2-3 arc minutes - when I push on the eq mount 8 inces higher - stars move 10 - 15 arc minutes! So I need to tighten things!

So its all a learning experience for me - I'm learning the slow way - reading, experiementation askling questions here - twere I to go to an observing night I think I pick up heaps - so I look forward to that!

[Glenhuon (Bill)]

I've also been very frustrated by the conflicting advice re drift alignment. The one here on IIS is the easiest to follow but I did have to reverse all the directions to get things going the right way. I'm not blaming the instructions, thats me
Given up for the moment and checking that everything on the mount is up to scratch before I try again. Patience is a virtue

Cheers
Bill

[h0ughy (David)]

yes I have seen many sites like that - and read it but the information given by Mr Ponders still sticks in my mind (after the 39th lesson LOL)

[g__day (Matthew)]

Well too I goofed the East / West correction - the diagram lefy me confusing the Meridan with the zenith - a real d'oh moment there!

Once I had that addressed I drift aligned taking 10 minute duration shots last night until I saw no DEC drift with the mount pointing low East and then with it North on the meridan. All of my drift test shots were the mount in polar aligned mode synch'ed on just one guide star. Calibrating Maxpoint with a 48 star model said I am within Az 58 arc seconds and Alt < 2 arc minutes from the SCP.

But I do see slight RA drift - so that is the next bug bear to really hone in on. I'll be interested to see if running the mount in a 3 star sync'd polar unaligned mode - then switching to polar alignment mode improves tracking by improving the alignment variables within the hand controller.


After that I'll try MaximDL to see if it manages RA drift any better than PHD. I really would like to conquer this RA drift once and for all! I am exhaustively going through PHD's many variable settings just to see if I can brute force the optimal settings.

With pinpoint polar alignment I'd love to see if RA drift almost dissappears.

Lots to try and fine tune still!

[AJames]

Originally Posted by g__day

Isn't it funny how something authorative sound in print is can be treated like holy writ? I remember an Australian Astronomy website that had rough polar alignment advice saying true South was about 11 degrees West of magnetic South for Sydney. I asked my dad - a retired surveyor and he said no its the opposite - due East. A google again identified two points of view - which I alerted the website owner too - he checked and corrected his advice!

If you are after the definitive explanation of this, then go to;
http://www.ga.gov.au/oracle/geomag/agrfform.jsp

This explains that the magnetic variation "D" is;

"D, the magnetic declination (sometimes called the magnetic variation), is the angle between the horizontal component of the magnetic field and true north. It is positive when the compass points east of true north, and negative when the compass points west of true north. Declination is given in degrees and its annual change is in degrees per year.
The value of magnetic declination should be added to a magnetic compass bearing to yield the true north bearing."

The calculation example on this page says;

"Let us say the magnetic declination for Perth (31 57'S, 115 51'E) at 1 July 2002 is -1.6 degrees. A compass bearing of 72 degrees in Perth converts to a true bearing of 70.4 degrees [72 + (-1.6)].
Map and compass users often require the angle between grid north and magnetic north. Grid north differs from true north by the "grid convergence". The MGA94 grid convergence for the Perth location above is -0.6 degrees. A true bearing of 70.4 degrees in Perth converts to a grid bearing of 69.8 degrees [70.4 + (-0.6)].
Grid convergence and magnetic declination are shown in diagrammatic form on some topographic maps. The signs of these values can be deduced from the diagram."

You can also calculate the magnetic variations for any location in Australia (on this same webpage) using the provide calculator, just providing the latitude and longitude, and height above sea level.

However, importantly, the magnetic variation is significantly different from place to place - in Australia and throughout the world.

Ie. In Sydney, 18th Jan 2008, the variation is "D" is 12.598 deg, while in Perth this is "D" -1.492. If you were on Macquarie Island in the Southern Ocean, the variation would be about 32.5 degrees. At Casey Station in Antarctica, the variation is -91.7 - meaning geographic north points to magnetic west ! [The message here is very clear : Don't get lost in Antarctica!"]

A suitable map for the variations can be obtained from; http://www.ga.gov.au/image_cache/GA5824.jpg

The error is so very easy to make. In astronomical calculations it is more frequent to place longitudes that are east of Greenwich are given as negative Ie. -151 deg East.

For all geophysical data, the latitudes east of Greenwich are usually positive (as it is with the Australian Geophysics website!)

The error in the statement you quote is just a matter of the direction.

Magnetic variation "D" is either;

- It is positive when the compass points east of true north
- It is negative when the compass points west of true north
- It is positive when the compass points west of true south
- It is negative when the compass points east of true south

The error in the statement you provide is the reverse direction being referred from magnetic south. In truth the statement should be,
as some references state;

"...about -11 degrees West of magnetic South for Sydney"
(interpreted as)
"...about +11 degrees East of magnetic South for Sydney"

Anyway. I hope this helps.

Andrew James

NOTE: What really should be stated to avoid the confusion completely is that the azimuth should always be quoted - measured in degrees from north through east. This avoids the whole east-west dilemma.
For example, the azimuth of magnetic south in Sydney, is 180 - 12.598 or 168.402 degrees from north.

COMMENT: Using this method for alignment is only approximate anyway.

I do think it is better if you want to approximately find true south, then just walk a couple metres away from the telescope and just look where the mount is actually pointing!

[If you don't know where south is, then you can use the Southern Cross by drawing a line through Acrux and Gacrux, measure 4.5 times the distance towards the celestial pole, and drop this place to the horizon. This is south.]

Visually you should be able to get within two degrees without much trouble. Then return to the telescope, and move the mount to this direction. Repeat if necessary. THEN, if you must use a compass, adjust the direction by the difference of the magnetic variation to make a better refinement.

If your do require more accuracy, then just use stars (by the many methods] to refine the direction of the mount.